Amanda,
There is one flaw in your request. When you release the voucher, you are releasing the prepayment. There is only one GL transaction entered. If you look at the GL Transactions tab on the voucher, they are the same as the ones on the journal transaction for the prepayment.
With respect to your issue with the approval map, you might be able to come up with some condition on the approval step that will allow it to not require approval if it see this value. Payment Ref does not contain ‘JrnlVch’ as sort of a secret word to avoid approval. I don’t know enough about your system or approval map.
I hope this helps.
John